The present invention relates to the production of magnesium metal by the reduction of magnesium oxide at elevated temperatures in the presence of an aluminum metal reducing agent and a molten oxidic slag, in an electric furnace, and the condensation of vaporized magnesium in a condenser.
An advantageous method of producing magnesium lies in the chemical reduction of magnesium oxide with a reducing agent in the presence of a molten oxide slag, in an electric furnace. A variety of metallothermic processes, as they are commonly called, for the production of magnesium employing various reducing agents, various types of reactants, and under various conditions of temperatures and pressures have been proposed.
In general, the various metallothermic processes are concerned with the production of metallic magnesium by the reduction of magnesium oxide with a metal reductant at elevated temperatures. Magnesium oxide, usually in the form of calcined dolomite (dolime) or calcined magnesite or mixtures thereof, is caused to react with a metallic reducing agent, such as silicon, aluminum, calcium or mixtures or alloys thereof, in the presence of a molten slag bath in a furnace at temperatures in excess of 1300.degree. C., to release magnesium vapor which may be condensed and collected. Some of the processes are carried out in the presence of an inert gas.
An early process of this type, called the Pidgeon process provides for the production of magnesium by the reduction of magnesium oxide with ferrosilicon.
A more recent process is exemplified by U.S. Pat. No. 2,971,833 and is known as the Magnetherm process. The process is operated under a very high vacuum (pressure ranging between 5 and 20 millimeters of mercury) and at a temperature of about 1500.degree. C. utilizing an electric furnace. Silicon is employed as the reductant, preferably in the form of ferrosilicon containing 70-80 percent Si, silicon of a purity in excess of 97%, or an aluminum-ferrosilicon. Care is taken to avoid that the silicon content of the residual ferrosilicon drops below 33.5%. A magnesia containing substance is dissolved in a liquid slag consisting essentially of lime, silica and alumina wherein the ratio of calcium oxide to silicon dioxide and the ratio of aluminum oxide to silicon dioxide are controlled.
Processes employing aluminum metal as the reductant are exemplified by U.S. Pat. Nos. 3,782,922, 2,527,722 and 2,527,724. The first patent discloses the production of magnesium by the reduction of magnesium oxide from a mixture of magnesium oxide and calcium oxide with a reducing agent comprising at least 85% aluminum in the presence of a molten calcium-aluminate slag bath at a temperature of about 1300.degree.-1700.degree. C. and a pressure of about atmospheric. Slag is removed when the magnesium oxide content thereof is less than five percent, with the slag comprising 35-65 percent aluminum oxide, 35-55 percent calcium oxide and 0-10 percent silicon dioxide. The other two patents utilize magnesium silicate in pure or ore form and aluminum as reactants. Dolomite is also used in the former and the magnesium silicate is of a particular granular size in the latter.
U.S. Pat. No. 3,658,509 also discloses the use of aluminum as the reductant, as well as silicon and aluminum-silicon alloys with the latter being preferred. In such process, an inert gas is used to obviate at least in part the need of a high vacuum, and the slag contains 20-50 percent silicon dioxide.
The primary object of this invention is to provide a process for production of magnesium metal wherein aluminum metal is reacted with a calcium aluminate magnesium slag, which eliminates the disadvantages of the prior art processes, while retaining the benefits thereof.
Another object of this invention is to provide a process for production of magnesium metal at atmospheric pressure by reaction of aluminum metal with a calcium magnesium aluminate slag, or with magnesium oxide in the presence of such slag, wherein the silicon dioxide content of the slag is maintained at a low level.
Another object of the invention is to provide a process for producing magnesium metal utilizing aluminum metal as the reductant, wherein magnesium vapor is produced at a pressure of one atmosphere thereby eliminating the need for vacuum equipment and permitting a continuous operation at such normal pressure.
An important object of the present invention is to provide a process of the foregoing type which is economical and which can utilize aluminum metal scrap as the metal reductant.
Other objects and advantages of the present invention will become more readily apparent from a consideration of the description hereinafter.